The present invention relates generally to magnetic resonance imaging (MRI) scanners, and, more specifically, to cryostats therein.
An MRI scanner includes a superconducting electrical coil for generating a strong magnetic field for diagnostic imaging of a target by magnetic resonance thereof. The coil is disposed inside a cryostatic vessel which includes liquid helium for achieving the cryogenic operating temperatures required for maximizing performance of the superconducting coil.
Maintaining superconducting low temperature of the coil requires suitable thermal insulation which is provided in part by surrounding the cryostatic vessel with a thermally insulating shield. The shield, in turn, is disposed inside a vacuum vessel for providing additional thermal insulation.
For maximizing performance of the MRI scanner, the cryostatic vessel must be precisely aligned both radially and axially inside the surrounding thermal shield. And, this must be accomplished with minimal interconnections therebetween to prevent undesirable thermal short circuits which would degrade the thermal isolation of the cryostatic vessel.
The cryostatic vessel is in the form of a tubular outer shell having integral endplates at axially opposite ends thereof. The endplates include center apertures joined to a tubular inner shell extending through the vessel defining its bore. The vessel is suitably sealed for containing therein the superconducting coil and the liquid helium.
The vessel is typically radially suspended concentrically inside the thermal shield by a plurality of radial suspensions or mounts at the axially opposite ends. The thermal shield is typically a cylindrical shell initially open at both axially opposite ends thereof during the manufacturing process for permitting nesting of the vessel and shield and installation of the radial mounts. Each radial mount typically includes a threaded fastener, such as a bolt, which is adjustable for adjusting the radial position of the vessel inside the shield. Four radial mounts are typically provided at each end of the vessel in diametrically opposite pairs along the vertical and horizontal centerlines thereof. By adjusting the bolts, corresponding lengths of the radial mounts are adjusted for permitting concentric alignment of the vessel inside the shield at both axial ends.
Axial suspensions or mounts are also provided between the vessel and shield typically at the middle thereof on diametrically opposite sides. The axial mounts extend axially with a radial inclination between the outer surface of the vessel and the inner surface of the shell, and also include threaded fasteners, such as bolts, for adjusting length and tension therein. The axial mounts are typically arranged in pairs extending in opposite axial directions so that the mounts may be adjusted individually to precisely control the axial position of the vessel inside the thermal shield.
The thermal shield is enclosed at its axially opposite ends by a corresponding pair of endplates, each having a central aperture through which a tubular inner shell is later mounted for completing the thermal shield to fully surround the cryostat vessel.
The cryostatic vessel must not only be precisely centered radially within the thermal shield, but also axially therein with equal gaps or clearances between the corresponding endplates of the vessel and shield. Accurate axial positioning of the vessel inside the shield is typically accomplished by providing a plurality of access holes in each of the shield endplates through which a measuring ruler may be inserted for measuring the clearance between the endplates. Four access holes are typically provided in each shield endplate in diametrically opposite pairs at the vertical and horizontal centerlines. Precise axial clearance between the corresponding endplates is required at each of the four circumferentially spaced apart access holes at each end of the vessel.
The axial clearances are adjusted by adjusting the corresponding lengths of the axial mounts. However, the axial alignment process is difficult and time consuming since it is basically a random process which is conducted iteratively. When any one axial clearance at the corresponding access hole is too small or too large, adjustment of the several axial mounts not only affects the out of specification axial clearance being addressed, but other axial clearances as well. This has been the assembly process for one type of conventional cryostat used in commercial service for over a year.
Furthermore, adjustment of the axial mounts may also affect radial alignment since the vessel is suspended inside the shield by both the radial and axial mounts. In addition to axial adjustment of the position of the vessel inside the shield, further adjustment of the radial position may also be required. Once these adjustments are made within a suitable tolerance, the shield endplates may then be permanently affixed to the outer shell thereof, typically by providing a plurality of circumferential tack welds around the perimeter of each of the shield endplates and the adjoining portions of the shield outer shell.
Accordingly, it is desired to provide an improved cryostat apparatus and assembly for reducing alignment time and increasing accuracy of alignment.